The diamond form of carbon possesses many desirable physical properties such as hardness, chemical inertness, infrared transparency, and excellent heat conductivity coupled with very high electrical resistivity. Consequently, diamond is a material with many important technological applications such as in optical devices, semiconductors, heat sinks, abrasives, tool coating, etc. It can also be used as a high-grade, radiation-resistant, high-temperature semiconductor with obvious applications in many technologies. Thus, there is considerable incentive to find practical ways to synthesize diamond, especially in film form, for these many and varied applications.
Various methods are known for the synthetic production of diamond, including diamond in film form. In particular, the deposition of diamond coatings on substrates to provide films is known.
One class of the methods developed for synthetic diamond deposition is the low pressure growth of diamond called the chemical vapor deposition (CVD) method. Three predominant CVD techniques have found favor in the literature.
One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen, wherein the hydrocarbon content usually is varied from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750.degree. C. to 2400.degree. C. The gas mixture disassociates at the filament surface, and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is heated to a temperature in the region of about 500.degree. C. to 1100.degree. C.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase the nucleation density and growth rate, and it is believed to enhance formation of diamond films as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems: One is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
A third method in use is direct deposit from acetylene as a hydrocarbon-rich oxyacetylene flame. In this technique, conducted at atmospheric pressure, a specific part of the flame is played on a substrate on which diamond grows at rates as high as 100 microns/hour or more. See Y. Matsui et al., Japan Journal of Applied Physics, 101, 28, p. 178 (1989).
In general, processes for the chemical vapor deposition of diamond involve selection of operating parameters such as the selection of a precursor gas and diluent gases, the mixture proportions of the gases, gas temperature and pressure, the substrate temperature, and means of gas activation. These parameters are adjusted to provide diamond nucleation and growth on a substrate. Mixture proportions and conditions must provide atomic hydrogen to stabilize the surface of the diamond film and preferably minimize the deposition of graphite. Codeposition of graphite is more evident if the hydrocarbon (methane) concentration is increased above about 3%.
The CVD techniques provide diamond films grown in tension. The tensile stress can cause the film to crack or peel from the substrate during growth because of the thermal gradient through the substrate or during cool-down, as a result of differences in the coefficients of thermal expansion between the diamond and the substrate material. Thus, for example, where molybdenum substrates are often used, the thermal gradient through the substrate can cause the substrate to bow due to the thermal expansion difference between the hot and cold sides. In addition, the coefficients of thermal expansion of diamond and molybdenum are so different that compressive stress is induced in the diamond coating as the coated molybdenum is cooled from the temperature at which diamond deposition takes place.
The diamond film grows in tension due to defects, and the inherent tensile stress/intrinsic strain is proportional to the film thickness and the rate of diamond deposition. This compressive stress and the inherent tensile stress in diamond is at first accommodated by elastic strain in the diamond. This elastic strain energy is released when the diamond is separated from the substrate. If the stored elastic strain energy is much greater than the interface surface energy, the separation will start simultaneously at many places on the diamond substrate interface, and a large number of small pieces of diamond will be obtained in the catastrophic separation. Thin diamond coatings can usually withstand such stress, but thick coatings may cause partial separation of the diamond layer from the molybdenum substrate and/or catastrophic failure of the diamond layer in the form of severe cracking and fragmentation.
It is well known that CVD diamond tends to nucleate on certain substrate materials more readily than others and that good bonding to the substrate is necessary during the growth period, particularly when growing thick films, to avoid catastrophic release of the film as a result of the intrinsic strain therein. However, the diamond film can be so strongly attached to the substrate that at the end of the growth period, where there is a significant differential in thermal expansion between the diamond and the substrate, the diamond film may crack during cool down. The use of release agents will promote the eventual removal of the film from the substrate but may cause the diamond to be so poorly bonded during growth that it causes a catastrophic release thereof from the substrate.
It is desirable to produce thick CVD diamond films which are easily removed from a substrate, do not release prematurely during deposition, and do not crack upon cool down.